1. Field of the Invention
The invention pertains to ion analyzers, and more especially to the ion analysis of samples of insulating material.
Ion analyzers are habitually used for the analysis of solid samples. If the elements to be analyzed emit negative ions, the sample is negatively polarized and the negative ions emitted are analyzed. However, if these elements emit positive ions, then the sample must be positively polarized and it is the positive ions which can then be analyzed. For the samples in which insulating zones are formed, or for samples of insulating material, the emission of charged particles creates a surface charge on the surface of the sample which must be compensated for to obtain suitable images of the sample.
2. Description of the Prior Art
A method and a device for the ion analysis of samples of insulating material from negative secondary ions emitted by the sample is known through the French patent application No. 83 00538, filed on Jan. 14, 1983. The method for discharging the imaged field and the corresponding device consist in directing, towards the analyzed surface, the beam of primary ions to be analyzed as well as an auxiliary beam of electrons which exactly compensates for the positive charges which tend to be created on the surface of the sample and to disturb the images if no method is used to neutralize them.
For the analysis of samples of insulating material, an operation in which the analysis is based on the positive secondary ions created during ion bombardment, the target is positively polarized to push back these positive secondary ions created by the impact of the primary beam. One approach, similar to the one described above for analysis based on negative ions, would consist in replacing the auxiliary beam of electrons by an auxiliary monokinetic beam of positive ions: but this approach is not worthwhile for it entails an excessively complicated assembly owing to the fact that the magnetic field which would bring this ancillary beam to the axis of the ionic emission lens would have to be far more intense than the one used previously, and because its effect on the secondary ions could no longer be compensated for as easily. Furthermore, the surface of the insulating material would need to possess the tendency of developing, in all circumstances, a negative charge during the ion bombardment. However, this is not the case when the ion bombardment is done with positive primary ions. To obtain a compensation, it would therefore be necessary to bombard the surface of the sample either with fast neutral atoms or with negative ions, or again, it would be necessarhy to have an additional auxiliary beam of electrons. However, the system would then become more complicated.
Other methods have been proposed, all pertaining to the analysis of the positive ions emitted by the target: a metallic grid is deposited by evaporation on the surface. In addition to the beam of primary ions, the insulating zones of the sample receive secondary low-energy electrons emitted by the grid and high-energy electrons produced on the acceleration electrode and attracted by the sample.
This low electronic flux on the imaged field can, however, be used for the compensation provided that the bombardment density of the primary ions is sufficiently lowered while the total intensity of the beam of primary ions on the target is kept constant. It is then possible to obtain a state of electrical equilibrium, where the insulating surface is practically equipotential and where the difference in potential compared with the conductive film is virtually zero, by modulating the density of the beam of primary ions in the imaged field. This lowering has disadvantages as the secondary ion currents used for imaging are then low. Furthermore, the adjustment of the density of the primary beam is a delicate task since it is obtained only by making variations in the focusing of the beam of primary ions by affecting the excitation of the capacitors: however, since there are diaphragms in the primary column, this operation may also alter the total intensity of the primary beam reaching the target. Finally, the zone bombarded is unnecessarily increased and, furthermore, the new regions affected may emit in a different way, causing a modification in the flow of secondary electrons which tends to increase the adjusting difficulty.
The object of the invention is a method and corresponding device for the ion analysis of samples of insulating material, suited to the case where the image is formed from positive secondary ions, a method and device which do not have the disadvantages referred to above.
3. Summary of the Invention
The invention relates to a method for the ion analysis of samples of insulating material where positive ions are emitted under the impact of a primary bombardment beam of positive ions, these positive ions being accelerated by an electrostatic field created in an accelerating space between an acceleration electrode and the sample taken to a very positive potential as compared to this electrode, method wherein the positive charge created in the imaged field of the sample is compensated for by secondary electrons emitted by the acceleration electrode under the impact of particles transmitted by the sample, focusing means being provided in the acceleration space to send back the exact quantity of secondary electrons needed for compensation to the imaged field.
The object of the invention is also a device for the ion analysis of samples of insulating material, used to analyze positive ions emitted by the imaged field of the sample under the impact of a beam of positive primary ions, device comprising a source of primary ions, a primary column to bring these primary ions to the imaged field of the sample, a collecting device comprising especially an acceleration electrode and a mass spectrometer, further comprising an additional electrode placed between the collecting device and the sample, and capable of being polarized at an intermediate voltage between that of the acceleration electrode and that of the sample, and means to set this intermediate voltage.